The current invention is directed to an apparatus for measuring the weight and/or torque on a drill bit. More specifically, the current invention is directed to the measurement of the weight and torque on a drill bit operating down hole in a well, such as an oil well.
In underground drilling, such as gas, oil or geothermal drilling, a bore is drilled through a formation deep in the earth. Such bores are formed by connecting a drill bit to sections of pipe, referred to as xe2x80x9cdrill pipe,xe2x80x9d so as to form an assembly commonly referred to as a xe2x80x9cdrill stringxe2x80x9d that is suspended from a rig at the surface and that extends down to the bottom of the bore. The drill bit is rotated so that it advances into the earth, thereby forming the bore. In rotary drilling, the drill bit is rotated by rotating the drill string at the surface. In directional drilling, the drill bit is rotated by a down hole mud motor coupled to the drill bit; the remainder of the drill string is not rotated during drilling. In a steerable drill string, the mud motor is bent at a slight angle to the centerline of the drill bit so as to create a side force that directs the path of the drill bit away from a straight line. In any event, in order to lubricate the drill bit and flush cuttings from its path, piston operated pumps on the surface pump a high pressure fluid, referred to as xe2x80x9cdrilling mud,xe2x80x9d through an internal passage in the drill string and out through the drill bit. The drilling mud then flows to the surface through the annular passage formed between the drill string and the surface of the bore.
Depending on the drilling operation, the pressure of the drilling mud flowing through the drill string will typically be between 1,000 and 25,000 psi. In addition, there is a large pressure drop at the drill bit so that the pressure of the drilling mud flowing outside the drill string is considerably less than that flowing inside the drill string. Thus, the components within the drill string are subject to large pressure forces. In addition, the components of the drill string are also subjected to wear and abrasion from drilling mud, as well as the vibration of the drill string.
Reaction forces applied to the drill bit by the formation exert a variety of forces on the drill bit, including compressive forces operating in the axial direction, side forces and torque. The compressive force, referred to as the xe2x80x9cweight on bit,xe2x80x9d can be controlled by varying the degree of support provided by the rig. The torque exerted on the drill bit by resistance from the formation, referred to xe2x80x9ctorque on bit,xe2x80x9d can be controlled by varying the torque applied by the motor that rotates the drill bit or that rotates the drill string.
Information concerning the weight and torque on the drill bit can provide useful information for the drilling operator. For example, the weight on the drill bit affects not only the rate at which the drill bit advances into the formation but the rate at which the drill bit wears. In addition, weight on bit information can be used for directional control of the drill string. By applying more or less weight one can control the build rate of the drill string. The torque on bit provides information about whether the bit is advancing smoothly into the formation or bouncing into and out of contact with the formation.
In the past, the weight and torque on the drill bit has been measured by means of strain gages incorporated into the drill string. The output from these strain gages is digitized and then transmitted to the surface via mud pulse telemetry that is, by encoding the information into pressure pulses created in the drilling mud that propagate to the surface where they are sensed by a transducer and decoded.
Unfortunately, the strain sensed by such gages is effect by not only the weight and torque on the bit but by side forces imposed on the drill bit that impart a bending moment to the drill string. Consequently, conventional weight and torque on bit measurement systems suffered from inaccuracies.
Consequently, it would be desirable to provide an apparatus for measuring weight and/or torque on a drill bit that is relatively insensitive to changes in the bending moment applied to the drill string.
It is an object of the current invention to provide an apparatus for measuring the weight and/or torque on a drill bit that is insensitive to changes in the bending moment applied to the drill string. This and other objects is accomplished in an apparatus comprising (i) a drill bit, (ii) a drill string operatively coupled to the drill bit, the drill string having a section disposed proximate the drill bit, the section of the drill string defining a centerline thereof, (iii) at least first, second and third pockets formed in the section of the drill string, the pockets circumferentially spaced equidistantly around the section of the drill bit, each of the pockets forming at least first and second walls, (iv) a first set of strain sensors for each of the pockets, each of the first sets of strain sensors affixed to one of the walls of its respective pocket, each of the first sets of strain sensors comprising first, second, third and fourth strain sensors circumferentially spaced equidistantly around the one of the walls of its respective pocket, each of the first strain sensors in each of the first sets of strain sensors disposed opposite the third strain sensors in its respective set, each of the second strain sensors in each of the first sets of strain sensors disposed opposite the fourth strain sensor in its respective set, each of first and third strain sensors in each of the first sets of strain sensors disposed along a first line approximately parallel to the centerline of the section of the drill string, each of the second and fourth strain sensors in each of the first sets of strain sensors disposed along a second line approximately perpendicular to the centerline of the section of the drill string, (v) first circuitry connecting each of the strain sensors in the first set of strain sensors, the first circuitry forming a first bridge, the first bridge comprising first, second, third and fourth legs, a first input junction formed between the first and second legs, a second input junction formed between the third and fourth legs, a first output junction formed between the first and fourth legs, a second output junction formed between the second and third legs, each of the first strain sensors in each of the first sets of strain sensors connected in series along the first leg of the first bridge, each of the second strain sensors in each of the first sets of strain sensors connected in series along the second leg of the first bridge, each of the third strain sensors in each of the first sets of strain sensors connected in series along the third leg of the first bridge, and each of the fourth strain sensors in each of the first sets of strain sensors connected in series along the fourth leg of the first bridge, (vi) means for applying a voltage across the first and second input terminals of the first bridge, (vii) means for sensing a voltage across the first and second output terminals of the first bridge, (viii) means for determining the weight on the drill bit from the voltage sensed across the first and second output terminals of the first bridge, (ix) a second set of strain sensors for each of the pockets, each of the second sets of strain sensors affixed to one of the walls of its respective pocket, each of the second sets of strain sensors comprising fifth, sixth, seventh, and eight strain sensors circumferentially spaced equidistantly around the one of the walls of its respective pocket, each of the fifth strain sensors in each of the second sets of strain sensors disposed opposite the seventh strain sensors in its respective set, each of the sixth strain sensors in each of the sets of strain sensors disposed opposite the eighth strain sensor in its respective set, each of first and third strain sensors in each of the second sets of strain sensors disposed along a third line oriented approximately 45xc2x0 to the first line, each of the second and fourth strain sensors in each of the sets of strain sensors disposed along a fourth line oriented approximately perpendicular to the third line, (x) second circuitry connecting each of the strain sensors, the circuitry forming a second bridge, the second bridge comprising first, second, third and fourth legs, a first input junction formed between the first and second legs, a second input junction formed between the third and fourth legs, a first output junction formed between the first and fourth legs, a second output junction formed between the second and third legs, each of the fifth strain sensors in each of the second sets of strain sensors connected in series along the first leg of the second bridge, each of the sixth strain sensors in each of the sets of strain sensors connected in series along the second leg of the second bridge, each of the seventh strain sensors in each of the sets of strain sensors connected in series along the third leg of the second bridge, and each of the eighth strain sensors in each of the sets of strain sensors connected in series along the fourth leg of the second bridge, (xi) means for applying a voltage across the first and second input terminals of the second bridge, (xii) means for sensing a voltage across the first and second output terminals of the second bridge, (xiii) means for determining the torque on the drill bit from the voltage sensed across the first and second output terminals of the second bridge.